Some of the major goals of high speed packet access (HSPA) evolution include higher data rates, higher system capacity and coverage, enhanced support for packet services, reduced latency, reduced operator costs and backward compatibility. Meeting these goals requires evolutions to the radio interface protocol and network architecture. More specifically, meeting these goals has required a set of enhancements and architecture changes to layer 2 (L2), (i.e., radio link control (RLC) and medium access control (MAC)), functionalities.
Some of the L2 enhancements include flexible RLC protocol data unit (PDU) sizes, high speed MAC (MAC-hs) segmentation/concatenation and multiplexing. In universal terrestrial radio access (UTRA) Release 6 (R6), the acknowledge mode (AM) RLC entities can only use a fixed RLC PDU size. In addition, the MAC-hs sub-layer in the Node-B can only support concatenation of MAC-d PDUs. The L2 enhancements of UTRA Release 7 (R7) result in significant RLC/MAC changes of R6 features.
The changes to the enhanced MAC-hs (MAC-ehs) architecture on the UTRAN side include the addition of a logical channel identifier (LCH-ID) multiplexing (MUX) entity. The LCH-ID MUX entity multiplexes logical channels into a priority queue. The MAC-ehs architecture further includes priority queue segmentation functionality and multiplexing MAC-ehs payload units from different priority queues into a MAC-ehs PDU.
The changes to the MAC-ehs architecture on the wireless transmit/receive unit (WTRU) side include disassembly of the MAC-ehs payload units from the MAC-ehs PDU. Further, after re-ordering, the MAC-ehs payload units are forwarded to a LCH-ID demultiplexing entity. This LCH-ID demultiplexing entity routes the MAC-ehs payload units to the correct reassembly entity based on the logical channel identifier. The MAC-ehs architecture at the WTRU also includes a reassembly entity that reassembles segmented MAC-ehs service data units (SDUs) and forwards full MAC-ehs SDUs to the higher layers.
Currently, when radio bearers are setup or reconfigured via radio resource control (RRC) signaling, the information element (IE) “radio bearer (RB) mapping info” is present. The “RB mapping info” contains information about the RLC instance and transport channels corresponding to the radio bearer (RB).
New information elements (IE)s may be added to the IE “RB mapping info”, that indicate whether the logical channel of an RLC instance supports flexible RLC PDUs, or whether the MAC sub-layers supports MAC-hs or MAC-ehs. For the purpose of this invention we will call these IEs “downlink (DL) RLC configuration” and “DL MAC-hs configuration”. The MAC-hs configuration has to be the same across all RBs mapped to a high speed-downlink shared channel (HS-DSCH), otherwise an invalid configuration will result.
In HSPA, the high speed shared channels are monitored by a WTRU in a single cell, (i.e., the serving high speed downlink shared channel (HS-DSCH) cell). Due to mobility, when the WTRU is moving from one cell to the other, the WTRU needs to perform a serving cell change by switching to a new serving HS-DSCH cell and terminating communication with the old serving HS-DSCH cell. In a Node-B relocation procedure, an inter-Node-B handover occurs from an old Node-B (i.e., a source Node-B) to a new Node-B (i.e., a target Node-B).
At the time of a serving Node-B change, the target Node-B needs to start transmission of data over the new configuration. The handover can occur within evolved HSPA Node-Bs which support the L2 enhancements, or to/from cells with or without L2 enhancements. For both cases, the WTRU must be able to perform a handover, adjust to the new configurations, and minimize data loss.
In a conventional system, (i.e., R6 system), when a handover occurs, a radio resource control (RRC) message can carry a MAC layer reset indicator. Specifically, when an inter-Node-B or intra-Node-B handover occurs, the data in the MAC-hs in the source Node-B is deleted, and the MAC-hs in the WTRU has to be reset. Upon reception of the reset indicator, the WTRU will perform the following sequence of functions:                1) flush a hybrid automatic repeat request (HARQ) soft buffer for all configured HARQ processes;        2) stop all active re-ordering release timer (T1) and set all timer T1 to their initial value;        3) start transmission sequence number (TSN) with a value 0 for the next transmission on every configured HARQ process;        4) initialize variables RcvWindow_UpperEdge and next expected_TSN to their initial values;        5) disassemble all MAC-hs PDUs in the re-ordering buffer and deliver all MAC-d PDUs to the MAC-d entity; and        6) flush the re-ordering buffer.        
With the introduction of new L2 enhancements, new procedures need to be defined in order to optimize and minimize data loss during a handover between R7 cells, or between an R7 cell and an R6 cell. Specifically, procedures that deal with resetting of the MAC-hs entity need to be modified in order to account for the new L2 enhancements.
In addition, it cannot be assumed that all of the R6 Node-Bs will be upgraded at the same time to R7 Node-Bs. Therefore, handovers between R6 and R7 cells may frequently occur. Due to the functional changes of the RLC and MAC, methods to perform handovers with minimal loss of quality and data between these cells must be defined. Specifically, on the WTRU side, the MAC-hs and the RLC must perform functional changes during the handovers.